A cryogenic liquid, such as liquid hydrogen, liquid oxygen and the like, is used as fuel for a rocket engine. Thus, a fuel tank that can withstand cryogenic temperatures plays an important role in aerospace applications. A conventional airtight tank is made of a metal; thus, its weight is heavy and manufacturing cost is high. For this reason, a composite material which is more durable and lighter than a metal has been a prime candidate for the new generation fuel tank material. However, a composite material comprised of an epoxy resin reinforced with a fiber, upon contact with a cryogenic liquid, has many micro cracks in the epoxy resin due to the difference in thermal expansion between the epoxy and the reinforcing fiber, thereby causing a fuel leak through the cracks.
In order to solve the above problem, Japanese Kokai Laid-open Publication No. 2002-104297 disclosed a technology to manufacture a light, airtight tank for holding a cryogenic liquid by bonding airtight liquid crystal polymer films with the use of an adhesive to form a liquid crystal polymer layer on the inner surface of the tank made of a composite material. Further, Japanese Kokai Laid-open Publication No. 2002-212320 disclosed a composite material, improved by means of a certain epoxy resin composition, which is resistant to crack formation even at cryogenic temperatures.
Note here that, according to the technology disclosed in the above Japanese Kokai Laid-open Publication No. 2002-104297, the liquid crystal polymer film is first cut into small pieces; these pieces are partially stuck together at the edges with respective adjacent pieces so that they can be bonded to each other through an adhesive layer; and the pieces are further bonded to the inner surface of the tank with the use of the adhesive to form the liquid crystal polymer layer.
The repetition of filling up a tank with a cryogenic liquid and discharging it means a frequent change in temperature between cryogenic and normal, thereby causing frequent contraction and expansion of the composite material comprising the tank. Thus, for the case of the tank with the liquid crystal polymer layer and the adhesive, cracks are generated in the adhesive layer connecting the liquid crystal polymer film pieces because of the difference in thermal expansion between the composite material and the adhesive. This poses a new problem, i.e. a fuel leak through the cracks in the adhesive layer. This problem occurs similarly even when the improved composite material, as disclosed in Japanese Kokai Laid-open Publication No. 2002-212320, is employed for fabricating the tank.
The entire disclosures of Japanese Kokai Laid-open Publications No. 2002-104297 and No. 2002-212320 are incorporated herein by reference.